1. Field of the Invention
The present invention relates to a water-treating process for preventing the formation of corrosion, scale and contamination on the surface of a metal that is in contact with water, particularly for preventing the formation of corrosion, scale and contamination in or on industrial heat exchangers, metal tubes in a cooling water system, surfaces that contact flowing water, the heating surfaces of boilers, heat-distillation type sea water desalting apparatus, and dust collectors and associated piping in a dust collecting system in iron and steel manufacturing industries.
2. Description of the Prior Art
Water fed to industrial heat exchangers and other cooling water systems contains a small quantity of suspended substances and slightly soluble inorganic salts such as carbonates, sulphates and silicates of calcium and magnesium.
In a cooling water system having a cooling tower and a heat exchanger, the circulating water is cooled by evaporation of a part of the water passing through the cooling tower. Owing to the evaporation, dissolved inorganic salts and suspended substances in the feed water are condensed in the circulating water and are deposited onto the heating surface of the heat exchanger to form scale thereon. Furthermore industrial heat exchangers, cooling water systems or the like made of carbon steel corroded by the attack of dissolved oxygen, especially the scale and portions having contamination adhered thereto corrode due to the formation of an oxygen concentration cell. Adhesion of such scale, contamination and corrosion products brings about a lowering of the heat transfer rates of heat exchangers and decreases the circulation rate of cooling water owing to increase of the pressure loss, that is, a lowering of the cooling efficiency, and further serious difficulties such as clogging of tubes of heat exchangers, penetration accident of tubes caused by local corrosion thereof, etc., which possibly lead to a temporary stoppage the operation of the apparatus.
Various kinds of scale inhibitors have been used to prevent the precipitation and adhesion of scale and contamination as aforesaid. As such scale inhibitors there are known lignin derivatives for instance, sodium ligninsulfonate, inorganic phosphorous compounds such as tri-poly phosphoric acid, pyrophosphoric acid, hexametaphosphoric acid, etc., organic phosphorous compounds such as alkyl phosphate, alkyl phosphite, etc. and polyacrylates. However, each of them is deficient in that the lignin derivatives, being natural products, are not stable in quality and are liable to color, and inorganic or organic phosphorous compounds cause the formation of scale in the presence of polyvalent metal ions such as calcium ions, and polyacrylates are inferior in the scale inhibiting effect, especially in inhibiting the formation of phosphatic scales.
As corrosion inhibitors chrome (VI) compounds are used, but they involve problems of danger to health and environmental pollution. In order to overcome such a problem, there have been utilized inorganic polyphosphoric acids, phosphonic acids, organic phosphoric acid esters, and polyvalent metal salts, such as Zn (II) ions and, Ni (II) ions. However inorganic polyphosphoric acids, phosphonic acids and organic phosphoric acid esters, when they are used in low concentration, adversely act so as to enhance corrosion, and when added in high concentrations, lead to the formation of scale. In more detail, the inorganic polyphosphoric acids are hydrolyzed in water to produce orthophosphoric acid ions which act upon calcium ions to form insoluble precipitates.
Phosphonic acids and organic phosphoric acid esters are hydrolyzed in cooling water and act upon calcium ions in an alkaline cooling water to form insoluble precipitates which turn into scales. As to the polyvalent metal ions, it is difficult to maintain a predetermined concentration of polyvalent metal ions in an alkaline cooling water, where calcium hardness coexists and the pH is high, and the polyvalent metal ions precipitate as hydroxides, phosphates, phosphonic acid, etc. Such phenomena vary depending on the cooling water temperature, and become greater when the calcium hardness increases and the pH rises. Therefore, it is necessary to determine the guantities of corrosion inhibitors added depending on the cooling water temperatures, as well as utilizing a cooling water which is low in calcium hardness and has a controlled pH. In an industrial cooling water systems, however, it is impossible to prevent corrosion and formation of scales simultaneously and efficiently, because the systems have many heat exchangers in which water temperature are respectively difficult, and the systems use cooling water having a high concentration, which contains relatively high calcium hardness.